There are two basic methods of positioning read/write heads in magnetic disk drives using closed loop feedback circuits. The first method, called dedicated servo, dedicates one head and one disk in the disk drive to the task of acquiring position information. The dedicated head is referred to as the servo head and the dedicated disk is referred to as the servo disk. In this method, all the data heads and disks must maintain their positions relative to the servo head and the servo disk or off-track error will result. Off-track errors reduce read margins and may cause data errors. The servo head is in a closed loop feedback system with the servo disk. The other data heads and other disks in the disk drive do not have the benefit of a separate closed loop feedback system. Thus, in order to function properly the other disks and heads must maintain their original position with respect to the servo head and the servo disk over time, through temperature extremes and after mechanical wear.
The second method of positioning read/write heads is called embedded servo. Embedded servo uses position information written on the surface of each disk in the disk drive. Each disk and head have loop feedback circuit. This eliminates the requirement of the dedicated servo method that each data head maintains its position with respect to another data head through temperature extremes, over time and after mechanical wear.
However, many other disadvantages are associated with the embedded servo method. For example, embedded servo reduces soft sectored formatting and requires position information to be recorded on each disk. The position information on each disk reduces data capacity and increases drive complexity. Therefore, it is desirable to develop an apparatus and method in which all the benefits of dedicated servo can be maintained by compensating for slight distortions caused by wear over time and by temperature fluctuations.